1. Field of the Invention
The present invention relates to a photomask, which is employed for working a workpiece such as a semiconductor wafer through a lithographic technique.
2. Description of the Prior Art
FIG. 9 is a schematic block diagram of an exposure apparatus including a photomask which is generally used in a photolithographic process, and FIG. 10 is an enlarged view of its essential part.
As shown in these FIGURES, light L.sub.1 output from a light source 1 is focused by a lens system 2 and emitted onto a photomask 3. In the photomask 3, a light shielding pattern 5 is formed on one major surface of a transparent substrate 4, so that light entering a region corresponding to the light shielding pattern 5 is out off within light L.sub.2 which is incident upon the photomask 3, while light entering the remaining region is transmitted. Light L.sub.3 selectively transmitted through the photomask 3 is collected within a resist film 8 which is formed on a substrate 7 serving as a workpiece, through a projecting lens system 6 of a magnification m, for example. Thus, the resist film 8 is partially photosensitized so that the mask pattern is transferred onto the resist film 8.
The conventional photomask 3 is formed in the aforementioned manner, and light L.sub.3 selectively transmitted through the photomask 3 forms an image on a focal plane 9 including a converging point P (plane perpendicular to the optical axis of the lens system 6 through the converging plane P) through the lens system 6. In this case, a prescribed depth of focus DOF expressed in the following equation is recognized in the lens system 6 with respect to light of a wavelength .lambda., assuming that NA represents its numerical aperture: ##EQU1##
Since the resist film 8 is substantially photosensitized within the range of the aforementioned depth of focus DOF about the focal plane 9 along the thickness direction of the resist film 8, it is preferable that the value of the depth of focus DOF is sufficiently large with respect to the thickness of the resist film 8.
With recent refinement of LSI, however, the numerical aperture NA tends to increase in consideration of the converging ability of the lens system 6. Consequently, the depth of focus DOF tends to decrease to the contrary. In a current manufacturing process for LSI, a lens system 6 having a numerical aperture NA of about 0.54 may be used for ultraviolet rays of 436 nm in wavelength .lambda.. for example, and in this case, the depth of focus DOF is about 1.5 .mu.m from the above equation (1). On the other hand, the thickness of the resist film 8 is also about 1.5 .mu.m, substantially equally to the depth of focus DOF. Therefore, accuracy is required for the exposure apparatus in order to correctly locate the resist film 8 within the depth of focus DOF, while accuracy is also required for an operation for setting the workpiece, and a defectively photosensitized region is caused in the resist film 8 upon merely a slight reduction of such accuracy.